A phase-change type optical disk has a recording layer on a transparent substrate. The recording layer comprises a material which changes its phase between amorphous state and crystalline state upon irradiation with laser power and which is capable of erasing old information and recording new one at the same time only by changing laser power (hereinafter, overwriting).
Chalcogen alloy such as In-Se alloys, In-Sb-Te alloy and Ge-Te-Sb alloy (U.S. Pat. No. 4,670,345) have been generally used as a recording material in phase-change type recordable and erasable optical disks. According to overwriting, some areas of a recording layer are irradiated with a high power laser beam to become amorphous since the recording materials are heated rapidly at their melting point or more and cooled rapidly to form recording marks; and the other areas of the recording layer are irradiated with a crystallizing laser power and are crystallized since the recording materials are heated below their melting point and cooled gradually to form erased areas.
Such overwriting is carried out by irradiating optical disks to a laser beam while the disks are rotated at a specific linear velocity on a drive unit. In order to complete the erasing by one time laser passing, the crystallizing speed of the recording material, i.e., transformation speed between amorphous state and crystalline state, must be higher than the crystallizing of recording marks per time required for a laser beam to pass through a certain point on an optical disk. Since the time required for a laser beam to pass through a certain point on an optical disk depends on a rotary speed (linear velocity), the crystallizing speed of the recording material must be higher than the rotary speed. When the crystallizing speed is increased or a crystallizing temperature is decreased so as to increase an erasing speed in regulation of the conditions for overwriting, recording marks (amorphous areas) formed by recording gradually transform into a crystalline phase having a lower energy level and are deteriorated during long term reading. As a result, the recording marks disappear and reliability of the recording data is lost.
Conventionally, transition metals have been added to a chalcogen alloy, or an amount of Ge in Sb--Te--Ge alloy has been increased, in order to avoid deterioration of recoding marks by long term reading. In the former process, though the addition of some 1 at % of transition metals increases stability of the recording marks, though it decreases erasing speed. Therefore, materials used particularly as media for recording at a high speed are limited, and contrast of reflectance is reduced because of a small difference in optical constant between amorphous and crystalline areas. In the latter process, as the amount of Ge increases, the melting point of the recording layer rises and the recording sensitivity is unfavorably decreased.
When optical disks having a recording layer of a chalcogen alloy are recorded and erased, protective layers are generally arranged on, directly under, or on both sides of the recording layer by deposition or sputtering in order to prevent a substrate from deforming with heating while recording and erasing, and to prevent the recording layer from deforming and oxidizing. Oxides, carbides, nitrides, fluorides, and sulfides of metals or semimetals have been known as materials for protective layers. Of these, ZnS is especially known for its strong adherence toward a recording layer. However, a protective layer made only from ZnS is insufficient in heat resistance since crystalline grains of ZnS are coarsened by repeated recording and erasing in overwriting.
Japanese Patent Application Laid-Open No. 103453/1988 discloses an optical disk which is enhanced in heat resistance of protective layers and is improved in the reduction of the cyclability caused by heat deformation of recording layers since glassy materials such as SiO.sub.2 are added to ZnS. In order to reduce the degradation of recording layers due to movement of recording materials caused by laser beam irradiation, Japanese Patent Application Laid-Open Nos. 263627/1991 and 152738/1991 propose to add oxygen or nitrogen, and hydrogen to the protective layers, respectively. However, conventional optical disks are insufficient in reliability of cyclability so that there has been a great demand for improving cyclability.
The present invention has been completed taking note of the drawbacks the conventional art has. An object of the present invention is to provide a phase-change type optical disk showing an excellent cyclability and less deterioration of recording marks formed by recording, which optical disk is attained by including a specific amount of sulfur in a recording layer comprising an alloy of specific components and/or in protective layers arranged on and directly under the recording layer. Another object is to provide a process for producing such a phase-change type optical disk.